1. Field of the Invention
The present invention relates to a semiconductor device and more particularly, to a semiconductor device having improved wirings through which a high-frequency electric current flows.
2. Description of the Related Art
FIG. 1 shows a conventional semiconductor device having two-layer wirings.
In FIG. 1, N.sup.+ -diffusion layer 202 is formed in a surface area of a P-silicon substrate 201 and a insulator film 203 is formed on the surface of the substrate. The insulator film 203 has a contact hole 204 at a position corresponding to the diffusion layer 202.
A silicon dioxide film obtained by the Chemical Vapor Deposition (CVD) or the Plasma-Enhanced CVD technique is generally used as the insulator film 203.
There is provided on the insulator film 203 with a first interlayer insulator film 206 in which first wirings are disposed, and a second interlayer insulator film (not shown) is formed on the film 206. The film 206 has first openings 224a and 224b in which the first wirings are arranged respectively. The opening 224a communicates with the contact hole 204 of the insulator film 203 so that the first wiring in the opening 224a is electrically connected to the N.sup.+ -diffusion layer 202 disposed thereunder. The film 206 also has a through hole 214 communicating with the first opening 224b so that the first wiring in the opening 224b is electrically connected to a second wirings disposed thereon. The openings 224a and 224b each has a depth nearly equal to half the thickness of the first interlayer insulator film 206.
In FIG. 1, the first wirings arranged in the respective openings 224a and 224b extend parallel to each other and perpendicular to the second wiring.
As the first interlayer insulator film 206 and the second interlayer insulator film, inorganic insulator films such as a silicon dioxide film which are obtained by the Plasma-Enhanced CVD technique, organic or inorganic insulator films obtained by application, and complex or combination films composed of these films.
The first wirings are respectively composed of three films, or a titanium/tungsten alloy film 205a, a platinum film 215 and a gold film 207. The gold film 207 act as a main component of each first wiring.
The second wiring is composed of two films, or a titanium/tungsten alloy film 205b and a gold film 217. The gold film 217 act as a main component of the second wiring. The first wiring in the opening 224a is electrically connected to the N.sup.+ -diffusion layer 202 in the substrate 201, and the first wiring in the opening 224b is electrically connected to the second wiring.
An aluminum or aluminum alloy film may be used instead of the gold film 207 and/or the gold film 217.
In the opening 224a, the titanium/tungsten alloy film 205a is contacted with the side face of the insulator film 203 and the surface of the diffusion layer 202 in the contact hole 204.
In the opening 224b, the titanium/tungsten alloy film 205a is contacted with the surface of the insulator film 203 and the top face of the gold film 207 is contacted with the bottom face of the titanium/tungsten alloy film 205b.
The surface of the second interlayer insulator film is covered with a passivation layer 213, which is generally composed of an organic insulator film obtained by application such as polyimide or the like.
In the case of first, second and third wirings being formed, there is provided in the second interlayer insulator film with a through hole similar to the through hole 214, through which the second wiring is electrically connected to the third wiring formed on the second interlayer insulator film. In the case of fourth wiring or more being formed additionally, a through hole or holes are provided and the electrical connection or connections are done similarly.
With the conventional semiconductor device, when it is operated by a high-frequency electric current, for example 80 GHz, most of the current flows in the peripheries of the gold films 207 and 217 of the first and second wirings due to the "skin effect", so that there is a problem that the operating speed is reduced since electric current densities in the centers of the gold films 207 and 217 become very small and apparent cross sectional areas thereof are reduced to increase their wiring resistances.